In a vehicle, a twist-beam or twist-axle is often used as a rear suspension structure. A twist-axle has two trailing arms for connecting road wheels to a vehicle's frame and a cross-beam member linking the trailing arms to form an integral body. Each trailing arm is pivotally connected to a vehicle's frame. When the road wheels are unequally displaced relative to the vehicle's frame, such as when the wheels encounter an uneven surface or when the vehicle is turning, the unequal displacement causes the trailing arms to pivot by different amounts, thereby resulting in twisting of the cross-beam member. The inherent torsional stiffness or resistance of the cross-beam member provides a restoring force to the unevenly displaced wheels.
For ride comfort and controllability (i.e., handling) of the vehicle, twist-axles generally need to meet compliance requirements for torsional stiffness. By compliance of torsional stiffness, it is meant that a twist-axle needs to have a torsional stiffness within a specified range. On the other hand, a twist-axle is a load bearing component and must be designed to have sufficient strength to support linear loads, such as static weight of a vehicle and dynamic load created as the vehicle moves.
There have been many proposals to make twist-axles that meet both torsional or roll stiffness and load bearing requirements. For example, there have been proposals to make twist-axles incorporating a separate torsion bar. The torsion bar in this design provides the required torsional stiffness or resistance. In some designs, such as those described in International Publication No. WO 2006/096980, a torsion element welded to a cross-beam member replaces the torsion bar to provide the required torsional resistance. Separate parts allow separate design requirements to be met. These proposals, however, require additional manufacturing and material costs. There have also been proposals to manufacture cross-beam members from a tubular blank, such as those described in U.S. Pat. Nos. 6,616,157 and 6,487,886. Such a cross-beam member has a mid-section of low torsional stiffness between two transition sections of high torsional stiffness, to achieve the overall torsional stiffness requirement. The mid-section has a U-shaped, V-shaped, or star-shaped double-walled cross-sectional profile of low torsional stiffness. However, as a twisting force is applied to such a cross-beam member, stresses tend to concentrate in the transition zones located between the mid-section and the end sections, which may cause durability concerns. Proposals have been made, such as that taught in U.S. Pat. No. 6,758,921, to selectively heat treat the transition zones in order to impart desired physical properties to the transition zones to prevent cracking. This approach, however, introduces additional manufacturing steps and also requires additional heat treatment equipment.
It is an object of the present invention to mitigate or obviate at least one of the above mentioned disadvantages.